Polymers represented by poly(meth)acrylic acid and polyvinyl alcohol contain in the molecules a group which is highly hydrophilic, and which readily forms a hydrogen bond. Such polymers have been publicly known as gas barrier polymers. Although, however, a film formed solely from such polymers exhibits excellent gas barrier properties against oxygen and the like under dry conditions, the film exhibits poor gas barrier properties against oxygen and the like under highly humid conditions due to hydrophilic nature of the polymers. In addition, such polymers have a problem with their resistance against moisture and hot water, since the polymers dissolve in hot water, and for other reasons. The poor resistance of such polymers against moisture and hot water has imposed restrictions on industrial use of the polymers as gas barrier resins.
Patent Literature 1 (claim 1 of Japanese Unexamined Patent Application Publication No. Hei. 10-316779) has proposed a method of producing a coating film in the following manner. Aqueous coating liquid containing at least a poly(meth)acrylic acid polymer, a polyalcohol polymer and an aqueous medium is applied to at least one surface of a film to be coated with the coating liquid. Thus, a coated film formed by arranging the coat layer on surface of the film to be coated with the coating liquid is fabricated. The coated film is stretched under heated conditions while the aqueous solvent exists in the coat layer.
Patent Literature 2 (claim 1 of Japanese Unexamined Patent Application Publication No. 2000-37822) has proposed another method of producing a coating film in the following manner. Aqueous coating liquid containing at least a poly(meth)acrylic acid polymer, a polyalcohol polymer, a plasticizer and an aqueous medium is applied to at least one surface of a film to be coated with the coating liquid. Thus, a coated film formed by arranging the coat layer on surface of the film to be coated with the coating liquid is fabricated. The coated film is stretched under heated conditions while the plasticizer exists in the coat layer.
Patent Literature 3 (Japanese Unexamined Patent Application Publication No. Hei. 10-231434) has proposed a composition formed from a carboxyl group containing resin which readily forms a hydrogen bond (specifically, a poly(meth)acrylic acid polymer), a hydroxyl group containing resin which readily forms a hydrogen bond (specifically, saccharides) and an inorganic layered compound.
Patent Literature 4 (Japanese Unexamined Patent Application Publication No. Hei. 10-237180) has proposed a resin composition containing a reaction product formed from a poly(meth)acrylic acid polymer, a polyalcohol polymer and a multivalent metal. Patent Literature 4 has disclosed that the resin composition is a gas barrier resin composition characterized by including, in the chemical structure, ester bonds formed through a reaction between the poly(meth)acrylic acid and the polyalcohol and ionic bonds formed between the poly(meth)acrylic acid and the multivalent metal ions. Patent Literature 4 has also disclosed that the mixture of the poly(meth)acrylic acid polymer and the polyalcohol polymer needs to be denatured by means of heat treatment or the like in order to sufficiently develop the oxygen-gas barrier properties and the resistance against high-temperature water vapor, hot water and the like.
Patent Literature 5 (Japanese Unexamined Patent Application Publication No. 2000-931) has proposed a film produced by applying a layer containing a metal compound on the surface of a molded product layer formed from a poly(meth)acrylic acid polymer and a polyalcohol polymer. Patent Literature 5 has disclosed that the aforementioned method makes it possible to easily obtain a film with enhanced gas barrier properties against oxygen and the like and an enhanced resistance against high-temperature water vapor and hot water.
However, Patent Literatures 4 and 5 have disclosed that the mixture of the poly(meth)acrylic acid polymer and the polyalcohol polymer needs to be denatured by means of heat treatment or the like in order to sufficiently develop the oxygen-gas barrier properties and the resistance against high-temperature water vapor, hot water and the like. In a case where the mixture is not sufficiently denatured, the existence of the polyalcohol in the mixture causes the obtained film to exhibit insufficient oxygen-gas barrier properties and insufficient resistance against high-temperature water vapor and hot water in spite of act of the multivalent metal compound. Furthermore, the Patent Literatures 4 and 5 have disclosed nothing about a stretched film which would be formed from the composition described in the literatures.
An object of the present invention is to provide the following stretched laminate film with oxygen-gas barrier properties and a method of producing the stretched laminate film. When a layer forming unit is defined as a unit where a layer formed from a polycarboxylate-based polymer containing a plasticizer and a layer containing a multivalent metal compound are adjacent to one another, the stretched laminate film includes a layer formed from a layer formed from a thermoplastic resin and at least one layer forming unit. In addition, in the stretched laminate film, at least the layer formed from the thermoplastic resin and the layer formed from the polycarboxylate-based polymer containing the plasticizer in the aforementioned layer forming unit are stretched.